For example, JP-A-2002-137054, JP-A-2000-54916, and JP-A-2007-51576 disclose a conventional EGR heat exchanger. In the conventional EGR heat exchanger, both ends of a tube, which is used as an EGR gas passage, in a flowing direction penetrate and are connected to header plates (also referred to as tube sheets), and exhaust gas and a cooling medium are separated from each other by the header plates. The EGR heat exchanger of JP-A-2007-51576 has a structure having a header plate (a side surface of a case) formed integrally with a gas tank portion and a water tank portion.
Further, JP-A-2007-225190, JP-A-2007-232355 corresponding to US 2007/0193732, and JP-A-2007-232330 disclose a heat exchanger in which exhaust gas and a cooling medium are separated from each other by stacking and joining tubes, a part of each of which is expanded, without using a header plate.
The heat exchanger of JP-A-2007-225190 has a header-plate-less structure configured by stacking tubes, end portions of which are expanded, and a gas tank portion and a water tank portion, which are integrally formed, are attached to the stacked tubes. However, a portion at which thermal strain may be generated has only a thickness corresponding to one plate.
The heat exchanger of JP-A-2007-232355 has a header-plate-less structure configured by stacking tubes, a part of each of which is expanded, and a gas tank and a water tank are attached to the stacked tubes. However, a portion at which thermal strain may be generated has only a thickness corresponding to one plate.
According to JP-A-2007-232330, in a joint portion between a gas tank and a water tank, the water tank is overlapped with the gas tank at one side surface among four side surface. However, the other three side surfaces do not have an overlapping structure that facilitates heat transfer.
Further, JP-A-2006-207887 corresponding to US 2008/0164014 discloses a heat exchanger with a particular slit block having a comb-like shape. According to JP-A-2006-207887, in a joint portion between a gas tank and a water tank, the gas tank is overlapped with the water tank on inner peripheral surfaces thereof at two side surfaces among four side surfaces. However, the structure of the heat exchanger becomes complicated, and each of the other two side surfaces has a thickness equal to a thickness corresponding to one plate.
As described in JP-A-2002-137054, JP-A-2000-54916, and JP-A-2007-51576, the heat exchanger with the header plate or an equivalent thereof has a complicated structure. Thus, a structure without a header plate or an equivalent thereof is desired. The problem can be solved by stacking tubes, end portions of which are expanded, to be the header-plate-less structure.
FIG. 12 shows a previously proposed heat exchanger, which is formed in view of JP-A-2007-225190, JP-A-2007-232355, and JP-A-2007-232330. In the heat exchanger shown in FIG. 12, a joint portion 9 (also referred to as a partition portion) is formed between a gas tank portion 7 which directly contacts exhaust gas and a water tank portion 8 which contacts LLC (coolant). In the joint portion 9, an end surface of the gas tank portion 7 is abutted on an end surface of the water tank portion 8 and the gas tank portion 7 and the water tank portion 8 are brazed in a T-shape.
It is assumed that high-temperature gas which can be up to 900° C. flows into the gas tank portion 7 of the EGR heat exchanger. The high-temperature gas in the gas tank portion 7 is heat-exchanged with a cooling medium at a temperature of approximately 100° C. in the water tank portion 8. Therefore, a temperature difference of approximately 200° C./10 mm is locally generated at the joint portion 9 between the high-temperature gas (high-temperature portion) and the cooling medium (low-temperature portion). Thermal strain is generated due to the temperature difference. If the thermal strain becomes excessive, a member configuring a flow passage is cracked and the fault of fluid leakage may be caused. It is required that such a crack can be prevented not only at one side surface but at least at four side surfaces, i.e., along an entire periphery of the joint portion 9.